Damascene processing is a method for forming interconnections on integrated circuits that involves formation of inlaid metal lines in trenches and vias formed in a dielectric layer (inter-metal dielectric). The metal conductive lines are formed by an electroplating process. Because copper or other mobile conductive material provides the conductive paths of the integrated circuit, the underlying silicon devices must be protected from metal ions (e.g., Cu2+) that might otherwise diffuse or drift into the silicon. Suitable materials for diffusion barrier include tantalum, tantalum nitride, tungsten, titanium tungsten, titanium nitride, tungsten nitride, and the like.
After the diffusion barrier is deposited and prior to electroplating, a seed layer of copper or other metal is typically applied by a physical vapor deposition (PVD) process to enable subsequent electrofilling of the features with copper inlay. In order to serve as a seed for electroplating, the seed layer should be continuous, stable and have good adhesion to the barrier layer. A barrier or capping layer may be deposited on the inlaid copper after it has been planarized via a chemical mechanical polishing process.
An integration issue when using Cu as the primary conductor in devices is electromigration. Electromigration redistributes the Cu in the line, and the resulting extrusions can expand into the dielectric space. Extrusions can cause the Cu metal to extend past the Cu line into an adjacent Cu line, thereby causing a short circuit.
As the dimension of semiconductor devices continues to shrink, the reliability of the device, particularly the electromigration of copper interconnects, becomes more challenging. This reduction of the electromigration performance is due to the increase of current density and decrease of the critical void dimension as the dimensions shrink.